The electrophotographic image forming process is well known. In image forming apparatuses such as a copying machine, a printer and a facsimile apparatus, the formation of the images is generally carried out through the electrophotographic process of forming electrostatic latent images on a photoreceptor. The photoreceptor is provided in its circumference with several image forming units such as a charging unit, a transfer roller, a developer unit, a cleaning blade, a cleaning brush and others.
Since the photoreceptor is generally brought into contact with these units, the surface of the photoreceptor (i.e., image bearing member) is worn away gradually with the rotation of the photoreceptor.
As a result, the thickness of a charge transport layer (which is the major part of the photoreceptor surface) gradually decreases and fluctuations in light exposure sensitivity may take place. It is considered that photo-induced discharge characteristics change with the abovementioned fluctuations and this causes further fluctuations in halftone image density. The photo-induced discharge characteristics are defined in the range from a uniform charging potential Vd down to a post-exposure potential V1, as will be detailed later on.
A measure for obviating the difficulty in the halftone image density may be contemplated. For example, the optimum amount of light exposure (or, optimum light exposure amount) L is determined for the thickness “t” of an image bearing member by (1) forming reference patterns of latent images on the surface of a photoreceptor at a predetermined timing under the conditions of both a uniform charging potential Vd and a developing bias Vb constant, and decreasing or increasing the amount of light exposure (or, light exposure amount) L by bits, (2) measuring the potential of the reference patterns with a potential sensor, and (3) based on the results from the potential measurement, adjusting the light exposure amount L such that a post-exposure potential V1 is brought close to a target post-exposure potential.
In this case, however, a drawback may be encountered, in which, since several latent image patterns have to be formed on the photoreceptor surface, another image formation can not be carried out over the period from initiating the formation of the latent image patterns to completing the determination of the optimum light exposure amount L for the film thickness t.
Japanese Laid-Open Patent Application No. 2002-244368 (application '368) describes a method, in which the thickness t of an image bearing member is estimated from a total of rotation and operating hours of, and number of copies made by, a photoreceptor. Thereafter, several parameters pertinent to suitable toner image density for the photoreceptor, such as a light exposure amount, a developing bias, and a charging bias are determined corresponding to the thickness t obtained as above.
Specifically, the optimum light exposure amounts L for the thicknesses, t1 through t5, for example, are computed in advance based on photo-induced discharge characteristics corresponding to the respective thicknesses, t1 through t5, and stored in a data storage unit in image forming apparatus as reference light exposure amounts corresponding to the respective thicknesses. The thickness t of an image bearing member is then estimated by a CPU in the apparatus from a total of rotation and operating hours of, and number of copies made by, a photoreceptor.
If the thickness t2 is obtained as the value of thickness by the CPU, one of the reference light exposure amounts corresponding to the t2 thickness is readout from the data storage unit, and thus readout amount is assigned to the optimum light exposure amount.
By adopting the method, which is described in the application '368, of reading out the optimum light exposure amount upon reaching a predetermined thickness for the photoreceptor in place of the aforementioned method of forming reference patterns of latent images, the optimum light exposure amount can now be determined suitable to an arbitrary thickness without forming the reference patterns.
Therefore, the aforementioned difficulty, in which another image cannot be formed over the period from initiating reference pattern formation to completing optimum light exposure determination for the thickness t, is considered to be obviated to a certain extent.
However, several problems are yet to be solved in the method of the application '368.
Namely, a plurality of discrete thickness values are stored in the data storage unit and the change in light exposure amount is made when the thickness of the image bearing member reaches one of the thickness values as described above. That is, no change in light exposure is feasible during the change from t1 to t2, for example. As a result, the problem of undue fluctuations in halftone toner density on the photoreceptor still exists over the period of the change.
In order to obviate the above noted difficulties, a method is contemplated in the present invention, in which a means is incorporated into the image forming apparatus to be capable of computing optimum light exposure amounts suitable to respective film thicknesses in place of the aforementioned method of storing optimum exposure values suitable to respective film thicknesses.
Then, the optimum light exposure amount can be computed by the present method to be suitable to the thickness estimated at a predetermined timing. Accordingly, it is considered that the difficulty mentioned above concerning no change of light exposure amount during the change from t1 to t2 can be obviated by the present method.
It may be noted that the technology is well known previously to improve image qualities over time and alleviate the effects from the environmental change, which is achieved by forming reference patterns of toner images on the surface of the photoreceptor, measuring the amount of toner adhered to the surface, and changing the developing bias Vb and uniform charging potential Vd based on the results obtained from the measurement.
Even in the case when the method of the application '368 is adopted and the optimum light exposure amount corresponding to the thickness is obtained after changing the uniform charging potential Vd, fluctuations may take place in toner image density on the photoreceptor.
This difficulty is considered due to the fact that photo-induced discharge characteristics of the photoreceptor may change with uniform charging potential Vd.
Namely, since the light exposure amount is obtained in this case based on the photo-induced discharge characteristics of the image bearing member corresponding to the film thickness without taking the effects of the above-noted change in the uniform charging potential into consideration, it is considered that the above difficulty is caused concerning the fluctuations in toner image density on the photoreceptor.